In a transmission system of a radio apparatus, a digital pre-distortion (DPD) circuit can be used for compensating non-linear distortion of an amplifier. The DPD circuit compares the input and output signals of the amplifier, for example, thereby detecting a signal difference generated by distortion, and multiplies the input signal of the amplifier by a coefficient having characteristics that cancel the signal difference.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2002-77284
[Patent Document 2] Japanese Laid-open Patent Publication No. 2001-16283
[Patent Document 3] Japanese Laid-open Patent Publication No. 2006-94486
For comparing the input and output signals of the amplifier, the DPD circuit branches a part of the output signal of the amplifier by use of a coupler or the like. Here, if an unnecessary high-frequency signal, which is radiated from other circuit parts to space, is coupled to the branched signal, it can occur that sufficient distortion compensation cannot be performed because of an error generated in detection of the signal difference. The previous techniques do not pay a consideration to the generation of an error caused by such coupling of the undesired wave.